Vapour vacuum pumps



April 26, 1960 ow 2,934,257

VAPOUR VACUUM PUMPS Filed Jan. 24, 1957 2 Sheets-Sheet 1 V k I1 BA D'ray HWFR 7 hrennn April 26, 1960 B. D. POWER VAPOUR VACUUM PUMPS 2 Sheets-Sheet 2 Filed Jan. 24, 1957 F IG. 3.

/III Unite VAPOUR VACUUM PUMPS Basil Dixon Power, Crawley, England, assignor to Edwards High Vacuum Limited, Crawley, England, a British company Application January 24, 1957, Serial No. 636,015

Claims priority, application Great Britain January 25, 1956 Claims. (Cl. 230-101) This invention relates tovapour vacuum pumps in which vapour from a heated working fillld passes to nozzles from which vapour jets emerge and serve to entrain air or other gas from a system or vessel to be exhausted.

It is frequently desirable in such pumps to interpose a bafiie or baffles between the pump mouth and the system being pumped in order to limit the extent to which vapour can enter the system being pumped. The baffles are usually water cooled but a greatly improved degree of freedom from pump fluid vapour in the system can be obtained by applying some refrigeration to the bafiie parts so that the vapour pressure of fluid condensed on the bafiie is reduced and this technique is frequently used where freedom from vapour contamination in the system is important.

The refrigeration referred to is usually achieved by circulating a refrigerated liquid in pipes or ducts connected to the baffle parts or by circulating refrigerator working fluid directly through the baffle ducts or by the use of a liquified gas or solid carbon dioxide. These methods are all to some extent cumbersome, particularly when applied to small plant Where the price and size of refrigerating equipment may be excessive in relation to the plant as a whole. Again, the use of liquid gas or solid carbon dioxide may be undesirable because of the .regular replenishment of the cooling medium which is necessary.

In a pumping system including a suitably backed vapour pump with a refrigerated baflie or cold trap in.

front of the vapour pump it is desirable that, if atmospheric air is admitted to the system, the cold baffle or trap be warmed before admission of air, or very quickly after admission of air, at least to a temperature above the prevailing dew point, or atmospheric moisture may condense copiously on the cooled baffle surfaces and this will provide an extra load on the pumps and may tend to contaminate the fore pump when pumping is resumed. Such quick warming up is difficult with conventional refrigerated baffles and cold traps, unless special heaters above the trap or baffle is not ideal, however, since most .high vacuum valves contain oil or grease for lubrication and sealing and elastomer gaskets for sealing and it is undesirable that the vapours from these substances should be able to enter the system without encountering the cooled baflie or trap.

The object of the present invention is to provide a ttes Patent C -rue hod of vcooling .baffles. which .is practicable 'becausel fiatenred Apr. 26, 1960 the bafile parts can be arranged to bevery well thermally insulated inside the vacuum region and the vapour quantities condensing on them are small so that a very small rate of heat extraction will suffice to chill them. The invention also provides a method of and means for heating the bafiies, or parts thereof, when desired, as, for example, for de-frosting purposes.

According to the present invention, for the purpose of cooling or heating baffles or portions thereof in a vapour vacuum pump, use is made of the Peltier cooling effect or the reversed current heating effect respectively, at one or more thermo junctions in good thermal contact with portions of the baffle.

For convenience it may be recalled that if by the application of an external potential difference, a current is compelled to flow through a thermojunction in the direction of the thermo-electric which would normally result from heating the junction, an extraction of heat or cooling effect is experienced at the junction which is known as the Peltier effect.

Any circuit involving a thermojunction inevitably includes a second thermojunction or its equivalent, and the current must flow through this in the opposite sense from that of its flow through the first junction. The second junction experiences a heating effect comparable to the cooling effect experienced by the first.

With conventional thermojunctions the heating and cooling effects described are very small and are to some extent masked by the ordinary resistance heating effect always present when a current flows in a practical circuit.

Developments in the field of semi-conductors, however,

have recently made available certain materials capable of providing cooling effects far more powerful than were previously possible-over 40 C. of refrigeration being obtainable by a suitable selection of materials, for example, by the use of p type and 11 type bismuth telluride .as referred to in Journal of Electronics, vol. 1, first series,

of machinery or the circulation of fluid. Their heat extraction capacity remains limited, but in utilising the effect in accordance with the present invention the limitation is comparatively unimportant for the reasons already given.

The application of the invention will enable water cooled diffusion pumps to attain lower ultimate pressures with greater freedom from working fluid vapour in the vacuum system than was previously possible without the use of expensive and complicated mechanical refrigeration equipment or of cold traps requiring the regular replenishment of a coolant, but a greater immediate benefit may prove to be its application to air cooled diffusion pumps.

Air cooled diffustion pumps which lend themselves very conveniently in many fields of application because of their freedom from the need for a water supply, have previously suffered from the disadvantage that they could not be cooled quite to room temperature by air cooling, so that a slow distillation of pumping fluid from the pump to the vacuum system, usually at room temperature, was often inevitable. The use of air cooled pumps is thus made impossible in all cases where system contamination cannotbe permitted and mechanically refrigerated 'baflies or trapsrequiring a coolant are uneconomic. I The invention makes possible the simple provision of compact cold baffies above such pumps, rendering their performance superior to that of similar water cooled pumps with Water cooled baflies and equivalent to that of water cooled pumps equipped-with similar cold baffles. Pumps b'odyin'g the invention may be built into compact mobile pumping sets dependent solely on an electrcity supply.

Various applications of the invention will now be described as examples with reference to the accompanying drawings in which:

Figure 1 shows an accessory for mountingahead of-a 'vapourvacuum p'umpand accommodating battles to be c'ooled and in which water cooling is employed,

Figure 2 shows a construction alternative to that of Figure 1 and in which air cooling-is employed,

Figure 3 shows a further construction "including illustration of air water cooling, v I

Figure 4 is a plan 'in section on the line X-X of Figure 3;

Figure 5 shows a particular construction of bathe element; and I Fig. 6 is a fragmentary detail illustrating one form of water cooling arrangement substitutable for the air cooling arrangement 28 in'the species of Figs/3 and 4.

Referring to Figure 1 of the drawings, a vapor vacuum um P provided with a cooling watercoiled pipe CP is surmounted by a bafile housing which comprises a body -=or pot 1 containing two separate rings of copper pipe 2 and 3 emerging separately from the pot through gland "seals 4 and 5 which are insulated by a suitable selection of gland material from the pot 1 'andtherefore from each other.

The battle assembly consists of a thick copper ring 6 'sup'porting'a number of annular copper discs 7 and one plain copper disc 8 by means of three copper struts 9 br'azed or soldered to the'copper ringand to the discs. The baflieasseinbly which is plated and polished to reduce radi'atio'n heat gain, is supported between the two rings of pipe 2 and 3 by a'nurnber of suitably spaced ribbons 'or strips of thermojunction material 10, 11, 12 each having the cold junction situated where'the strip is secured to the copper ring and in good thermal contact with the copper ring, for example as shown at 13.

Cooling water is supplied to the two'rings of pipe 2 "and 3, taking care that the'water doesfn'otprovide a too 'low'resistance electrical path between them, and a suitfabl'e from a-source S is also applied to thepipes. The thermo-couple hot junctions are maintained near cooling water temperature and the cold junctionsexperieri'ce'a refrigeratingefiect which'chills the copper ring *and baflle assembly in the desired manner.

Current is supplied to the rings 2 and 3 so that it will flow through the strips 10, 11 and 12 in parallel and if owing to an electrically conducting water supply, it is diflicult to insulate the two water cooled rings from each other, one only may be water cooled and the second replaced by a copper ring electrically insulatedfrom but thermally well connectedto the first ring.

The construction shown in Figure 2 is similar to that of Figure 1 but it will be seen that the rings of copper pipe 2 and 3 of Figure 1 have been replaced by two turned copper rings 14 and 15 which form part of the wall of the pot 1 and are electrically insulated from the pot and from each other by insulating gland rings 16,

17, 18 and 19 which also form vacuum tight seals at the various junctions. The copper rings 14 and 15 are cooled -by external cooling fins 20, 21 over which a draught of fair-may be directed if desired. The actuating froma source S is applied between these rings and the action is otherwise as described with reference to Figure 1. Simply for clarity in the drawing, the details of the baffle plate assembly and the pump P (Fig. 1) have been omitted from Figure 2.

A developed form of baffie cooling arrangement is shown in Figures 3 and 4'but'the principle of cooling employed isas already describedwith'referencedo Figuresl and 2. The construction shown in-Figures 3 and r4 intendedto meet a number of conditions which have to be taken into consideration because of thev'erysmall "heatext'raction capacity of the thermojunctions. Thus,

amount of incident radiation -may become excessive.

Again the cooled part of the bafileshould be designed so that, as far as possible, multiple re-reflections of radiant heat falling on the battle, which re-reflections lead to increased absorption of radiant heat are avoided and preferably the cooled part of the battle should have a radiation shield between it and the hot parts of the associated pump.

Referring now to Figures v3 and 4, copper hot junction terminal blocks 22 --'and 23 are :secured to the pot 1 by screws 24, electrically insulated gaskets 25 and gas sealing gaskets 26 being provided. Air cooling by vanes 28 is illustrated in Figs. 3 and 4, but it will be understood that the invention in its broader aspects is not limited to any particular type of cooling means, and that other cooling means, e.g. as bya pipe 27 (Fig. 6) for conveying cooling fluid, such as water, in thermal conducting relation with said thermojunction means 22, 29, may be employed-in lieu of the air cooling means shown in Figs. 3 and 4. The terminals T are connected to a suitable source of electrical supply as indicated at S. The accessory again is combined with a vapor vacuum pump as indicated at P, Fig. 3,'and more fully in Fig. 1.

Bars 29 of thermojunction material, for example bismuthtelluride, are sweated or otherwise strongly'secured to the blocks 22 and 23 and to a shallow cup shaped bafile 30 below which a radiation shield 31 is disposed.

The'working principles already described apply to the construction shown in Figures 3 and 4 and, while this construction enables all the desirable conditions outlined above to be met, it provides the further advantage that a stage of conventional bafiling, at room temperature or cooling water temperature, is provided by the radiation shield 31in conjunction with the walls of the bafile body before the back migrating vapour molecules from the pump reach the cooled bafile member at all. The amount of vapour to be condensed by the cooled ba'flle member is therefore reduced.

Inthe constructions described with'reference to Figures 3 and 4, the use of two small pillars 'or beams of rectangular section have been described, and it has been assumed that the two junctions to the common baflie part extract heat exactly as would a single junction and the shapes described are constructionally convenient.

A baffie assembly larger than that described would absorb more radiant heat from its surroundings and might therefore require a greater heat extraction rate. This can be achieved by using pillars of thermojunction material of greater cross section and providing a greater more electricallyinsulated sections each cooled by 'a single pair of pillars of thermojunction material and suitably to connect all the pillars in series. Sucha construction is shownin Figure 5 in which the bafile'element consists of sections 32, 33 and 34 mechanically connected together but insulated fromv each other by insulators 35.

Thermojunction elements 36, 37, 38, 3h, 40 and 41 are strongly attached to'the batilesections'as shown. 'This battle is mounted so that current supplied to the'thermojunction element 36 willlfiowviabafile section 32 to the element '37, thence direct toelement 38, via baffle section 33 to the element 39 Jand'from that element direct to element 40 and via baffle section 34 to element 41.

Cooled bafiies constructed :in accordance with thepresent-invention rare cooledby therrefrigera'ting actionprw duced when a current flows in an appropriate direction across a junction or junctions of suitable materials. If the direction of the current is reversed, then the analogous heating effect is produced quite apart from and in addition to the resistance heating ordinarily associated with current flow. The cooled parts of a baffle embodying the invention are preferably of light construction and may be rapidly heated up to ambient temperature, or if desired to a higher temperature within the safe limits for the constructional materials, merely by reversing the flow of the current and suitably adjusting its value. This facility for producing local heating is advantageous for de-frosting purposes, for example.

The dificulties, discussed earlier in this specification, associated with conventional cooled bafiies, are thus removed. It becomes practicable to mount a cooled bafile on the system side of the isolation valve whenever desired or to open an unvalved system incorporating a bafile to atmosphere with the knowledge that the bafile can be rapidly brought up to and held at a temperature which ensures freedom from the sort of contamination discussed.

Alternative constructional details and materials may of course be used in carrying out the invention, dependent upon specific requirements arising from the application of the invention to particular forms of pump.

I claim:

1. In combination, a vapor vacuum pump having an inlet end, a bafile at said inlet end, thermojunction means in thermal conducting relation with said baffle, and means connected with said thermojunction means for passing an electric current therethrough in such direction as to cool said baflle.

2, In combination, a vapor vacuum pump having an inlet end, a baflle at said inlet end, thermojunction means in thermal conducting relation with said bathe, and means connected with said thermojunction means for passing an electric current therethrough in such direction as to heat said bafile.

3. In combination, an assembly comprising a vacuum pump having an inlet end, a baflie at said inlet end, and thermojunction means in thermal conducting relation with said baflie, piping for conveying cooling water, said piping being in thermal conducting relation with said thermojunction means, said piping also being electrically connected to said thermojunction means and being connectable to a source of electrical power for passing current through said thermojunction means in such direction as to effect cooling of said thermojunction means.

4. In combination, an assembly comprising a vacuum pump having an inlet end, a baflie at said inlet end, and thermojunction means in thermal conducting relation with said bathe, piping for conveying cooling water, said piping being in thermal conducting relation with said thermojunction means, said piping also being electrically connected to said thermojunction means and being connectable to a source of electrical power for passing current through said thermojunction means in such direction as to effect heating of said thermojunction means.

5. In combination, a vapor vacuum pump having an inlet end, a bafiie housing at said inlet end, a baffle assembly within said housing, thermojunction means in thermal conducting relationship with said bafiie assembly, and means electrically connected with said thermojunction means for passing an electric current therethrough in such direction as to cool said baflie assembly.

6. In combination, a vapor vacuum pump having an inlet end, a baflie housing at said inlet end, a bafile assembly within said housing, thermojunction means in thermal conducting relationship with said baflie assembly, and means electrically connected with said thermojunction means for passing an electric current therethrough in such direction as to heat said baffle assembly.

7. In combination, a vapor vacuum pump having an inlet end, a baffle assembly and a housing for said bafile assembly at said inlet end, air cooled members supported from said housing, electrically insulated and vacuum tight seals between said air cooled members and said housing, at least one thermojunction means in thermal conducting relationship with said bafile assembly and said air cooled members, said air cooled members being electrically connected with said thermojunction means and being connectable to a source of power for passing electrical current through said thermojunction means in the direction to efiect cooling of said baflle assembly.

8. In combination, a vapor vacuum pump having an inlet end, a baffie assembly and a housing for said bafile assembly at said inlet end, air cooled members supported from said housing, electrically insulated and vacuum tight seals between said air cooled members and said housing, at least one thermojunction means in thermal conducting relationship with said bafile assembly and said air cooled members, said air cooled members being electrically connected with said thermojunction means and being connectable to a source of power for passing electrical current through said thermojunction means in the direction to effect heating of said bafile assembly.

9. In combination, a vapor vacuum pump having an inlet end, a baflle assembly and a housing for said bafile assembly at said inlet end, water cooled members supported from said housing, electrically insulated and vacuum tight seals between said water cooled members and said housing, at least one thermojunction means in thermal conducting relationship with said bafile assembly and said water cooled members, said water cooled members being electrically connected with said thermojunction means and being connectable to a source of power for passing electrical current through said thermojunction means in the direction to eifect cooling of said bafile assembly.

10. In combination, a vapor vacuum pump having an inlet end, a bafiie assembly and a housing for said batfie assembly at said inlet end, water cooled members supported from said housing, electrically insulated and vacuum tight seals between said water cooled members and said housing, at least one thermojunction means in thermal conducting relationship with said bafiie assembly and said water cooled members, said water cooled members being electrically connected with said thermojunction means and being connectable to a source of power for passing electrical current through said thermojunction means in the direction to effect heating of said baffle assembly.

References Cited in the file of this patent UNITED STATES PATENTS 2,777,975 Aigrain Jan. 15, 1957 

